1. Field of Invention
This invention relates generally to the field of optical communications and, more particularly, to an analog transmitter and external cavity laser suited for use in optical communication systems.
2. Description of the Prior Art
Analog transmitters for both broadcast and narrowcast (QAM-format) applications in cable television (CATV) commonly use directly-modulated distributed feedback (DFB) lasers or externally modulated sources (continuous wave (CW) DFB laser plus an external modulator). The choice is primarily dictated by the applications, by the operating wavelength (typically 1310 nanometer-nm or 1550 nm), by the reach and optical power level. The various transmitter options generally have both advantages and disadvantages, typically relating to a few fundamental issues of analog transmission. The factors considered in evaluating transmitter options typically include, for example, the level of the distortion produced at the output of the transmitter, distortion degradation due to the presence of optical filters and/or optical amplifiers (such as erbium-doped-fiber amplifiers, EDFA), and dispersion occurring as the signal travels through single mode fibers, especially at 1550 nm wavelength.
Wavelength Division Multiplexing (WDM) allows the simultaneous transmission of multiple laser beams, each at a different wavelength, inside a single strand of an optical fiber. WDM can be used in all types of fiber-based optical systems including baseband digital systems, analog broadcast and narrowcast systems, and RF-over-fiber systems. Increasing the number of wavelength channels that can be transmitted simultaneously led to the introduction of dense WDM (DWDM). Standardization bodies such as the International Telecommunication Union (ITU) have introduced standards for DWDM systems specifying among other things the spacing between neighboring wavelengths (also referred to as ITU grid or standard grid). As an example, I.T.U. G.957 is a standards document titled “ITU-T Specification: Optical Interfaces For Equipments And Systems Relating To The Synchronous Digital Hierarchy”.
Optical transmitters commonly employ methods to suppress stimulated Brillouin scattering (SBS). Simply stated, SBS is caused by a nonlinear optical-acoustic interaction inside single mode fibers. SBS can limit transmission reach (that is, transmission distance) and can also limit the maximum amount of optical power that can be launched into the fiber. Typical methods of SBS suppression include dithering of the transmitted signal with frequencies and format differing from those used for typical CATV transmissions.
Externally modulated transmitters seem to offer the best performance for many optical communication systems but at the expense of high cost, high optical coupling losses (typically about 5 to about 7 decibels, dB) and generally require sophisticated techniques for linearization of the analog signal.
Thus, a need exists in the art for improved optical transmission systems and components providing improved system performance, improved system reliability, and/or acceptable system performance at reduced cost.